Effects of climate change on litter decomposition in a mixed oak forest under decline

Abstract: Litter decomposition is a fundamental process for biogeochemical cycles in forest ecosystems. Among the main drivers of litter decomposition, several studies have shown that the key activity of soil fauna is modulated by climatic conditions, which implies that climate change is likely to modify this relationship. However, the magnitude and sign of this effect are yet unclear. Here, we performed a litter decomposition study for two Quercus tree species under natural conditions in a mixed forest in Southern Spain, where we established 6 permanent 15 × 20 m plots, and experimentally simulated a reduction in 30% of total rainfall in half of them. We used two types of litterbags with different mesh sizes to allow and exclude mesofauna, and filled them with dried litter of the two dominant tree species, Quercus suber and Q. canariensis. Litterbags were placed under every adult tree within the plots, and litter mass loss was estimated after 3, 9 and 18 months (2 bags, 3 times, 43 trees = 258 litterbags). We then performed a linear mixed-effect model to test the single effects and interactions between rainfall reduction, fauna exclusion and tree species on litter mass loss. Litter fauna was extracted using the Tullgren funnel method, identified and classified to trophic groups (decomposers and predators). Our results showed that the experimental rainfall reduction produced changes on faunal composition that could alter litter decomposition dynamics. Rainfall reduction, faunal exclusion and tree species significantly affected litter mass loss (litter mass × time interaction). We also detected some interactive effects between factors with interesting implications for litter decomposition. Unexpectedly, drier conditions accelerated litter decomposition rates of both Quercus species at some stages of the process, and the presence of mesofauna enhanced this process by 6%. Regarding tree species, the temporal dynamics were different, as Q. canariensis litter C decayed faster under rainfall reduction in the second part of the process in presence and absence of mesofauna. In contrast, Q. suber litter C decomposed faster at rainfall reduction in the first stages, decreasing its rates after 9 months. At the end of the experiment, the litter of Q. suber had larger C losses in control conditions, but the presence of mesofauna reduced these differences, showing its important role in modulating the relationship between climate and decomposition process. Our results suggest that climate change will have an important role in litter decomposition, potentially speeding-up biogeochemical cycles, but this effect is eventually influenced by soil fauna activity in litter decomposition.